The present invention relates to the field of secure data transmission and specifically to an improvement for steganographic methods of securely transmitting data between remotely located parties, such as set out in the applicant's co-pending patent application having publication number WO2011/161660.
Steganographic transmission methods comprise hiding ‘useful’ information within extraneous information. The useful information may be recovered by a recipient (e.g. a receiving terminal) if the position of the useful information within a received transmission signal is known.
The applicant's co-pending patent application having publication number WO2011/161660, which is incorporated herein by reference insofar as permitted by the relevant national law, provides an improved steganographic transmission method. The position of the useful information is provided by an identification variable. This identification variable is usually transferred along with the steganographic transmission signal to an intended receiving terminal. The identification variable may be expressed as one or more coordinates defined with respect to a terminal's selected coordinate measurement domain. The confidentiality of the hidden useful information is maintained provided that the coordinate measurement domain with respect to which the identification variable coordinates are defined is unknown to an eavesdropper.
An intended receiving terminal is able to recover the useful information comprised within the received signal if the receiving terminal is provided with a coordinate transform enabling conversion of the received identification variable coordinates into the coordinate measurement domain adopted by the receiving terminal.
For example, and in order to better illustrate the transmission method of the applicant's co-pending application, FIG. 1 illustrates a sending terminal 40 provided with a clock 42—Clock A—which clock measures irregular time intervals. In other words, the sending terminal's clock 42 measures the passage of time differently than a standard clock. For example, the sending terminal's clock may be accelerated, in which case the clock 42 may indicate a passage of time of five seconds, when in reality with respect to standard time, only one second has lapsed. Similarly, the sending terminal's clock 42 may be slower than standard time, in which case an indicated time lapse of one second may correspond to a time lapse of five seconds in standard time. In short, without knowledge of the sending terminal's adopted coordinate measurement domain, an eavesdropper 44 cannot make sense of the identification variable coordinate, and therefore cannot recover the useful information from the intercepted signal.
The receiving terminal 46 is provided with its own clock 48—Clock B. The receiving terminal 46 is provided with a coordinate transform matrix M which maps coordinate values, in this case time coordinates, expressed with respect to the sending terminal's adopted measurement coordinate domain (also referred to as a coordinate measurement frame), to time coordinate values expressed with respect to the receiving terminal's adopted measurement coordinate domain.
The transmitted message is forwarded from the sending terminal 40 to the receiving terminal 46 along with the coordinates {a,b} which define the position of the concealed useful information expressed with respect to the sending terminal's adopted coordinate measurement domain. For example, expressed with respect to the time domain of the sending terminal 40. Upon receipt of the message and associated coordinates {a,b} the receiving terminal uses the coordinate transform M to express the received coordinates in its own measurement coordinate frame. In the illustrated example of FIG. 1, this comprises expressing the received time coordinates with respect to its own clock 48.
The eavesdropper 44 is unable to recover the concealed useful information in the absence of a coordinate transform which converts coordinates expressed with respect to the sending terminal's confidential measurement coordinate frame, into coordinates expressed with respect to the eavesdropper's adopted coordinate measurement frame.
Such a described steganographic transmission method does not take into account signal distortion, including attenuation effects such as amplitude damping, and time distortion such as pulse broadening. Over short transmission distances the signal distortion may be negligible. However, over long transmission distances the effects may be noticeable, and may increase data transmission error rates. The reason is that the coordinate transform M may not compensate for signal distortion. The result is that when the received identification variable values are expressed with respect to the receiving terminal's selected frame of reference, the recalculated coordinate values may no longer accurately indicate the actual position of the useful information within the received signal. The introduced signal distortion effects, in particular distortion in the signal's time domain, is analogous to an introduced coordinate shift, which is not accounted for in the receiving terminal's adopted coordinate transform M.
Accordingly, the embodiments described herein provide a means and method for improving the fidelity of transmission methods and in particular the afore-described steganographic transmission method, and specifically provide a means for accounting for signal distortion effects acquired during signal transmission.